1. Field of the Invention
The present invention relates to a system for enabling surgery to be performed by vibrational heating and more particularly to a system for surgery by ultrasonic heating guided by magnetic resonance (MR) imaging.
2. Description of Related Art
Conventional Magnetic Resonance Imaging (MRI) provides the radiologist with internal views of a subject's anatomy. MRI provides excellent contrast between different tissues and is useful in planning surgical procedures. A tumor in a subject is much more visible in an MR image than as seen in actual surgery because the tumor and normal tissue often look similar in surgery. The tumor can also be obscured by blood during surgery. A view of the heated region is provided with the use of MR temperature sensitive pulse sequences. Known MR temperature sensitive pulse sequences are described in U.S. Pat. No. 4,914,608 Invivo Method for Determining and Imaging Temperature of an Object/Subject from Diffusion Coefficients Obtained by Nuclear Magnetic Resonance, Denis LeBihan, Jose Delannoy, and Ronald L. Levin issued Apr. 3, 1990. Experiments on animals show that a heated zone above a critical temperature destroys tissue. This zone increases in size with time as the heat is applied to reach a steady state of both temperature and heat flow. If the maximum temperature is limited to 100 deg. C, then the heated zone, the area exceeding a critical temperature causing destruction of tissue, approaches 1 centimeter in diameter. It is difficult to predict the heated zone geometry because the heat flow depends on the profusion of blood as well as the tissue thermal properties.
Tumors have been selectively destroyed in cancer subjects using focussed ultrasound heating in the absence of MR imaging at the University of Arizona, as reported by B. E. Billard, K. Hynynen and Robert B. Roemer Effects of Physical Parameters on High Temperature Ultrasound Hyperthermia Ultrasound in Med. & Biol. Vol. 16, No. 4, pp. 409-420, 1990 and hereby incorporated by reference. Billard et al. disclose that the control of heat is improved by using short heating pulses where the effect of blood perfusion is negligible. However, since they do not image the temperature distribution, it is difficult to hit small, deep laying targets.
It would be beneficial to be able to accurately localize heat to selectively kill or destroy tumor tissue without damage to surrounding healthy tissue.